The mining industry utilizes various devices to separate valuable minerals from host contaminants after extraction from the earth. Initially the ore preparation procedure involves crushing the run-of-mine rock from several feet in size down to approximately 1-3 inches. This preliminary crushing step is followed by one or more stages of grinding to reduce the ore to an average size less than 1 mm. These latter grinding steps typically use large rotating cylindrical mills containing a charge of spherical steel balls that are used as a grinding media. The balls are in a constant tumbling motion due to the rotation of the mill. The ore is fed into one end of the mill, it progresses through the grinding chamber and is discharged from the opposite end. As the ore progresses through the mill the grinding media impacts the material resulting in fracture and breakage of the individual pieces into smaller and smaller particles.
The tumbling motion of the ball charge can also result in fracture of the steel balls. Additionally, mechanical abrasion will wear the ball surface causing a reduction in size of the grinding media. The net result of this process is generation of various shapes of steel which are significantly smaller than the original spherical ball. A new grinding ball will typically range from 3 to 5 inches in diameter. The broken or worn ball components can be as large as a hemisphere of the original ball or fragments having dimensions of less than 3/4 inch. Depending on the mill design, these fragments will discharge with the mineral and report to downstream equipment.
The ball fragments cause two distinct problems in ore processing facilities. The first, and most notable, is wear on subsequent equipment. Grinding is typically a wet process and the ore/water matrix is pumped between various unit operations. The metallic fragments cause significant wear on pumps, piping and other downstream equipment. The costs associated with maintenance downtime and equipment rebuild can be substantial. Secondly, and less recognized, is the effect of the ball fragments on mill efficiency. The circuit design for most grinding operations is such that the ball fragments that discharge the mill will return with the new feed to the grinding circuit. As a result, a substantial build-up of fragments can occur in the grinding mill occupying volume that would otherwise be filled by mineral slurry. This loss in active mill volume can de-rate the mill capacity by as much as 10%. Furthermore, the small mass of the fragments does not provide a sufficient impact force to effectively fracture the mineral particles in the mill.
Applicant is aware of the following U.S. Pat. Nos. 2,332,701; 3,179,345; 3,856,666; 4,013,233; 4,062,497; 4,726,531; 4,781,821; 5,092,986; 5,110,056; 5,394,991; and, 5,462,234.
U.S. Pat. No. 2,332,701 recognizes the problem presented by worn and fragmented grinding media in ball mills. This ball mill continuously discharges grinding media with the ground material and return to the feed end of the mill only that portion of the grinding media which is in good condition and of the correct size. A trommel screen sorts the output and an elevator conducts the useful grinding media back to the feed end of the mill. This patent does not address the problem of separation of metallic contaminants from the slurry discharge of an operating grinding mill.